1. Field of the Invention
This invention relates to an engine control apparatus, and more particularly to an engine control apparatus installed in a vehicle having an oxygen sensor, an oxygen sensor output value of which varies in accordance with an oxygen concentration of exhaust gas.
2. Description of the Related Art
An oxygen sensor may be disposed on an exhaust path of a vehicle. Air-fuel ratio feedback control is performed in the vehicle on the basis of an output voltage of the oxygen sensor in order to adjust a fuel injection amount so that an air-fuel ratio of an engine reaches the stoichiometric air-fuel ratio. As a result, a purification performance of a three-way catalyst that purifies exhaust gas can be maintained.
The output voltage of the oxygen sensor varies according to an oxygen concentration of the exhaust gas. Further, the output voltage of the oxygen sensor exhibits a characteristic of varying rapidly about the stoichiometric air-fuel ratio. Using this characteristic, a determination can be made from the output voltage value of the oxygen sensor as to whether the air-fuel ratio of the engine is richer or leaner than the stoichiometric air-fuel ratio. A determination result is expressed by binary data based on whether the air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio. Air-fuel ratio feedback based on this binary determination result is implemented widely.
In recent years, as exhaust gas regulations become stricter, there is increasing demand for an improvement in the precision of air-fuel ratio feedback control. As described above, the output voltage of the oxygen sensor varies rapidly about the stoichiometric air-fuel ratio. More specifically, when the air-fuel ratio advances to the rich side of the stoichiometric air-fuel ratio, the output voltage of the oxygen sensor increases rapidly initially and then increases gently. When the air-fuel ratio advances to the lean side of the stoichiometric air-fuel ratio, meanwhile, the output voltage of the oxygen sensor decreases rapidly initially and then decreases gently.
Further, the characteristic of the oxygen sensor output outside the vicinity of the stoichiometric air-fuel ratio is affected greatly by variation in a sensor element temperature. When an oxygen sensor is used as an air-fuel ratio sensor, it is important to estimate the sensor element temperature of the oxygen sensor. Accordingly, an air-fuel ratio feedback method that includes detection or estimation of the sensor element temperature of the oxygen sensor has been proposed (see JP 4607163 B2, for example).
In a system configuration described in JP 4607163 B2, a three-dimensional oxygen sensor map is stored in advance in a memory of a control unit. On the oxygen sensor map, the sensor element temperature of the oxygen sensor is stored in association with an engine rotation speed and a throttle opening. The sensor element temperature of the oxygen sensor is estimated by reading the sensor element temperature from the map in accordance with operating conditions. The oxygen sensor output value is then corrected on the basis of the estimation result of the sensor element temperature. Further, an actual air-fuel ratio (referred to hereafter as the actual air-fuel ratio) is calculated from the corrected oxygen sensor output value. Hence, feedback control is performed on the basis of a deviation between the actual air-fuel ratio and a target air-fuel ratio (the stoichiometric air-fuel ratio). According to JP 4607163 B2, therefore, a large improvement in control precision can be achieved over conventional, widely implemented air-fuel ratio feedback control based on a binary determination result (i.e. whether the air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio).